1. Field of the Invention
The invention relates to apparatus for mounting a rotatable optical disk.
2. Description of the Prior Art
Planar rotatable disks are used to great advantage in various optical systems. For example, rotatable holographic disks known as a hologons are employed in a class of optical instruments known as beam scanners. The hologon is located in the path of a light beam and is spun at a high rate; the beam is thereby caused to deflect in a scanning motion.
Rotatable optical disks are also used as media for storing information. For example, storage systems that employ optically-based writing and reading apparatus are known as optical memories. An optical disk is spun in the path of a writing apparatus. The information to be recorded is used to alter one or more of the structural or electromagnetic properties of the disk. The disk may thereafter be read by again spinning it in proximity to a reading apparatus.
Glass is the material of choice for constructing many of the disks used in optical disk systems. The disk may be composed entirely of glass or the disk may comprise a glass substrate to which one of various surface coatings have been applied. The choice of glass for constructing optical disks is founded on several optical and structural properties that are well known in the art.
One property in particular, the yield strength of the optical disk, is pertinent to the present invention. Glass is inherently a very strong material: researchers have reported tensile strengths well in excess of 40,000 psi for pristine glass fibers that are protected from all adverse environmental influences. Unfortunately, the strength of glass in practice is 10-100 times lower than that of its pristine condition. The yield strength of an object formed from glass, such as an optical disk, is therefore but a small fraction of the aforementioned tensile strength figure because of stress concentrations introduced by surface defects.
These defects, which are have been termed Griffith flaws, usually occur in the form of small surface microcracks. These microfractures arise due to accidental mechanical damage that occurs during manufacture, processing, and use (cf. Ernsberger, F.M., Advances in Glass Technology. Part 1, pp. 511-524, Plenum Press, New York (1962); and Griffith, A.A., Philos. Trans. R. Soc., 221A, 163 (1921)). An applied load causes stresses in the vicinity of the crack tip. Fracture occurs on an atomistic level when the bonds between the glass atoms are stretched past the breaking point.
When a glass optical disk is rotated at a spin rate that is roughly in excess of 1,000 r.p.m., the disk is subjected to extreme stress induced by centrifugal force. Unfortunately, at such a high spin rate, the disk's ability to withstand the stress can be exceeded and the disk will fail (fracture). Failure occurs because the glass optical disks is characteristically unable to yield to the localized points of high stress concentration. That is, the disk yield strength is insufficient due to surface imperfections such as a one or more microfractures.
There are two conventional approaches by which practitioners of the prior art have attempted to preserve the high intrinsic yield strength of glass optical disks. First, the glass can be created with a flawless surface and thereafter the surface must be protected from damage. This may be done by acid polishing in a hydrofluoric acid solution and then protecting the glass with a plastic film. Usually this is impractical because of the expense and because some of the most outstanding advantages of glass, such as its optical clarity, are lost.
The second approach takes advantage of the fact that disk failure is always the result of tensile stresses. If the glass is pre-stressed so as to put the surface in compression, the resulting compressive stress has to be overcome before tensile stresses are encountered. Glass is categorically defined as an amorphous solid and is a product of fusion that has cooled to a rigid condition without crystallizing. Pre-stressing is achieved by quenching the glass from a softened condition so that the surface solidifies first. The core of the glass cools more slowly and in doing so it contracts and pulls the already rigid surfaces into compression. However, this approach can cause undesirable optical distortion of the disk.
Glass may also be strengthened by one of several chemical ion exchange processes. Typically ion exchange strengthening is carried out in molten salt baths. This is known as chemical strengthening, and it has the advantage that almost no distortion occurs as the result of the strengthening. Chemically-strengthened glass optical disks are therefore preferred over thermally-tempered glass optical disks, which invariably have some distortion. However, both processes are costly and complex. Special glass compositions, for example, must be formulated for chemical processing. Most chemically-strengthened glasses are derived from soda-aluminosilicate composition developed especially for chemical strengthening.
Furthermore, there is a need in some applications, such as in high-rate beam scanning apparatus, to rotate an optical disk beyond the safe revolution rate of a even the best chemically- or thermally-strengthened glass optical disks. Chemically-strengthened glass disks have been rotated (spun) at spin rates in excess of 20,000 r.p.m., whereupon the disks have failed dramatically (i.e., exploded) under the influence of the extreme centrifugal stress.